Method of impregnating and digesting cellulosic materials in a horizontal digestor



May 27, 1969 i INVENTOR amm E. sHlcK w mwvd TTORN EY S United StatesPatent O 3,446,698 METHOD OF IMPREGNATING AND DIGESTING CELLULOSICMATERIALS IN A HORIZONTAL DIGESTOR Philip E. Shick, Toledo, Ohio,assignor to Owens-Illinois,

Inc., a corporation of Ohiof Filed June 1, 1965, Ser. No. 460,371 Int.Cl. D21c 7/00 U.S. Cl. 162-19 2 Claims ABSTRACT F THE DISCLOSURE Thisinvention relates to :a process for continuously digesting wood chipswherein two connected |treating zones lare utilized. In the first zone,the -Wood chips are impregnated with a digesting liquor. Likewise, thefirst zone contains, in addition to lthe chips and the digested liquid,a non-condensed gas, said non-condensed gas generally keeping theItemperature of said reaction zone such that effective impregnation ofthe chips with the digesting liquor is possible. Under impregnation, thechips are then passed to ya second zone, the chips and digesting liquorare exposed to steam in such a fashion that the temperature is caused torise to ya point where digest-ion is effected. rl`he atmosphere of thefirst zone essentially comprises a non-condensed glas, while theatmosphere of the second zone comprises essentially steam.

The present invention relates to a novel Imethod and novel apparatus ofutility in the manufacture of paper. Most particularly, the presentinvention relates to the manufacture of paper pulp by means of digestionof ligno cellulosic substances such as wood chips. More specically,'thepresent invention relates to lsuch apparatus, methods, techniques andsystems embodying features of novelty in a particular lachievement ofpressure .above atmospheric in order to iachieve efficient impregnationof the cellulose source, usually -wood chips.

It has long -been recognized tha-t pressure impregnation of cellulosefiber-yielding substances resul-ts in improved yield as well as economyof pulping operations. The Pandia Division of Bllack-Clfawson Company ofHamilton, Ohio, markets a line of pressure digesters featuring multiple.re-actors in series having internal pulp-moving screws .and heat/pressure influencing components providing controlled yand timed movementof the, for example, wood chips through the heat `and pressure zone,thereby providing, it is alleged, 'im-proved impregna-tion land improvedpulp production eficiency.

The present invention relates generally to pressure pulp digester unitsand particularly to such units of improved performance in terms ofpulping eliiciency, both las to yield Iand improved character of theultimate pulp.

Pressure .tube digesting reactor units marketed by the Pandia Division-of B-lack-Clawson Company, can be modified to incorporate a v-alvebetween, for example, two vertically spaced horizontal tubes used forsequential movement of the wood chips `and the ultim-ate pulptherethrough whereby difference in temperature yand pressure conditionscan be maintained for each section providing selective control ofimpregnation and control of pressure Iand temperature conditions in eachzone in order to achieve adesirable :and complete pulping. The u-se ofthese inter-'mediate valves has proven expensive as to installation-andoperation. Additionally, operation has not 3,446,698 Patented May 27,1969 ICC proven eminently smooth or satisfactory in the yaccomplishmentof their designed ends.

Accordingly, it is an object of the present invention to provideimproved apparatus and methods which obviate the necessity of employingspecial intermediate valves in sequential, zoned, pressure tube reactorsfor digestion of cellulose-producing substances, e.g., Wood chips. v

It is a particular object of the present invention to provide a systemwhich in operation, both las 4to the appara- Itus and method, avoids thepresence, in the resultant pulp, of chips having incompletely cookedcenters.

It is still another object of the present invention to prov-ide apulping syste-m which is capable of producing -a pulp of considerablygreater brightness than known heretofore.

It is a particular object of the present inven-tion to provide a methodyand appara-tus which together combine and cooperate to give 'animproved pulping system as compared to those known heretofore.

I-t is still Ianother object of Ithe present invention to prov-ide apulping system which, in terms of method and apparatus, is capable offacile control Iand operation and yet is marked 'by pulp of greaterbrightness 'and repreu senfting a Imore efficient system in toto.

It is yet another object of the present invention -to provide a pulpingsystem including method and apparatus which in operation `prov-idesimproved impregna-tion of the cellulose source, e.g., the wood chip,such that the eficiency of pulping is markedly increased as to yield`and yals to quality of pulp produced.

The foregoing as well las other objects of the present invention willbecome apparent to those skilled in |the art from the following detaileddescription taken in conjunction with the Kannexed sheet of drawings onwhich there is presented, for purposes of illustration, a singleembodiment of the present invention.

In the drawings:

The iigure is a partially schematic, partially sectional side elevationview of an `array of pulping apparatus components shown in interrelatedoperative condition and thereby adapted to yield digested pulp ready forintroduction into a refiner, a defibrator or the like.

In its simplest embodiment, the presen-t invention contemplates thediscovery that improved impregnation can be achieved by a method andIapparatus which embodies features of construction :and operationproviding sepa- Irate pressure zones in an otherwise uni-tary pressuredigesting reactor tube system. By the expedient of selective locationfor :the introduction ofthe impregnating high temperature steam andselective location for introduction of a noncondensable gas medium, -anelevated pressure is achieved without accompanying elevatedtemperatures.

In a particular embodiment, the non-condensable gas is introduced,substantially contemporaneously with the wood chips, to a firsthorizontal reactor tube, defining a first zone, equipped withchip-transporting yhelical blades which propel the chips smoothly,uniformly and horizontally transversely therethrough to a connectconduit which in turn leads to a second horizontal reactor tube,defining a second zone, equippedwith chip and/or transporting helicalblades and said tube, defining a second zone, being Afurnished with asteam inlet arrangement for introducing, into contact with the nowpressure-impregnated chips, high temperature steam. Preferably,digesting liquor or the chemical or semi-chemical type, e.g., sulphite,acid sulphite, bisulphite or sulphate type,

etc., is introduced into the first reactor tube about contemporaneouslywith the introduced wood chips and the introduced non-condensable gas,whereby the impregnation, rapid neutralization and initial pulpingreactions take place under these conditions of high pressure but of lowtemperature.

An understanding of the present invention will be more apparent to thoseskilled in the art from the following more detailed discussion in whichreference will be had to FIG. l; which figure serves to illustrate boththe method and apparatus of the present invention.

In the figure, a feed hopper .11 is situated vertically' above and uidlyin connection with a tubular feeder 13 containing a rotatable helicalblade 14 extending from shaft 14a. The hopper 11 connects with thefeeder through a cylindrical vertical inlet opening 12. The feederincludes a converging wall section 15 fiuidly connecting with an inletpipe 18 which extends downwardly to connect with the first reactor tube20 situated horizontally, as illustrated. The outlet end 21 of thetubular reactor 20 connects fluidly with a vertical transfer conduit 23which connects with a tubular reactor tube 25 of the present invention,which is similar in construction to the reactor tube 20. The reactortube 25 includes an inlet end 26 which connects with the verticaltransfer pipe 23 at one end with a downwardly opening outlet pipe 27which connects with a pocket type rotary discharger 28, as shown.Returning to inlet 18, conduit 30 connects therewith opposite the feedhopper outlet 16 and connects via line 31, control valve 32, line 33,pressure regulator 34, line and valve 37, with a pressure cylinder 36 ofnitrogen. Line 38 connects with line 30 via T 39 and leads to a pressuregauge 40.

The tubular reactor 20 has connected thereto a line 41 connecting with apressure gauge 42 at the inlet end proximate inlet pipe 18. Atemperature gauge 43 is located at the outlet end 21 of reactor tube 20.A similar temperature gauge 44 extends into the inlet end 26 of tubularreactor 25 proximate the vertical transfer pipe 23.

Conduit 45 leads into the tubular reactor 20 proximate the inlet pipe 18and extends from the discharge side of a pump 46 mounted on base 47.Conduit 48 leads from a supply tank, 48a, of digesting liquor, via ametering pump, 4811, to the inlet end of centrifugal pump 46. Conduit 49leads from the lower segment of reactor tube 20, as shown, to connectwith conduit 48 leading to the liquor circulating pump 46. Conduit 50exits into vertical transfer pipe 23 and serves for introduction ofsteam proceeding through valve 51. Conduit 50 is equipped with apressure gauge 52.

The rotary discharger 28 includes a cylindrical shell 28a in whichrotates a hollow central tube 28b having a central perforate wall 28C.The element 28b in rotation connects oppositely disposed tubularconduits 54 and 56; the latter of which connects with an air supply.Thel rotatable tube .member 28b can also connect with the outlet 27whereupon the steam pressure in the system will ll one half of themember 28b with a supply of digested pulp, as shown. Rotation of thetube 28b, a one-quarter' turn, causes it to connect with outlet 54,whereupon a' blast introduced through conduit 56 will push the pulp bymeans of the perforate plate 28e outwardly to a blow tank, for example.Continued rotation of the tube 28b causes a repetition of the justdescribed exhaust procedure.

In operation, wood chips 61 of relatively uniform size are located inthe hopper 11 from which they drop through the outlet 12 into thecylindrical feeder 13. Blades 14 rotating on shaft 14a pressure pack thechips (by reason of the converging wall section 15) into plugs whichproceed through the outlet 16 onto the inlet pipe 18 Ifrom which thechips fall by gravity downwardly into the first tubular reactor 20. Theformation of the plugs serves as a pressure seal against loss ofpressure within the tube reactors 20 and 25. Rotating helical blade 20apropels the chips laterally to the outlet end 21. Contem- 4 poraneouslywith the introduction of the chips, a noncondensable gas, for example,nitrogen, is introduced through the conduit 30. Simultaneously, .aliquor, capable of attacking the lignins and encrustations or bindingcomponents holding the wood cellulose fibers together, is introducedthrough conduit 45 into intimate contact with the chips being propelledalong the reactor 20. From the outlet end, the Wood chips, substantiallyseparated from the liquor which exits through outlet line 49, passdownwardly through transfer pipe 23 into the lower tubular reactor 25wherein they are propelled reversely to the left by helical blade 25a topass downwardly out the outlet 27. As the chips drop through thetransfer conduit 23, they are contacted by inlet steam entering throughthe line 50. The digested wood pulp or exhausted chips pass thence outoutlet 27 into the pocket-type rotary discharger Where they aredischarged to a blow tank in conventional fashion. It is a feature ofthe present invention that the temperature within the tubular reactor 20can be Imaintained at a substantially lower temperature, for example,approximately 15 to 30 less, than the temperature maintained within thetubular reactor 25. This is accomplished by reason of the introductionof the steam into the transfer conduit 23, rather than into the firstreactor in .accordance with conventional practice. Contemporaneously andalso a controlling factor in the maintenance of the lesser temperaturein the first tubular reactor is the provision for introduction of anon-condensable padding gas, .as it were, such as nitrogen, through theinlet conduit 30 proximate the location at which the wood chips areintroduced into the tubular conduit reactor tube 20. The introduction ofthe non-condensable gas creates a positive padding pressure of said gas,precluding or reducing passage of the inlet steam (entering throughconduit 50) upward into the confines of the tubular reactor 20. Byreason of the introduction of the non-condensable nitrogen and also byreason of the passage of the wood chips via the helical blade 20a in thetubular reactor 20 `and the helical 'blade 25a in reactor 25, the steamis urged positively downwardly and then laterally to the left and isentrained with the liquorimpregnated wood chips.

p An apparatus as described in FIG. 1 was operated in accordance withthe present invention in order to determine the effect of theintroduction of a non-condensable gas. Chips utilized in the series ofexperiments consisted primarily of oak wood having a typical analysis asfollows:

Percent Lignin 25.2 Alphacellulose 45.7 Hemicellulose 23.2

The chips had a specific gravity of 0.521 and a moisture content of37.8%.

Example l Table I below lists the conditions including the following:the time of the run ranging from 0 to 29 minutes; the steam pressurereading of steam gauge 52; the pressure within the nitrogen supply tankin pounds per square inch gauge; the nitrogen pressure at the inlet tothe digester inlet pipe 18 at gauge 40; the temperature in the topreactor tube as taken on temperature gauge 43; the temperature in thebottom tube reactor 25 as recorded from temperature gauge 44; and theflow rate of the nitrogen in pound moles per minute.

6 T472-l. The brightness tests were conducted in accordance -with TAPPIStandard Method T452 M-58 using Photovolt Corporations Reiiection MeterModel 610. As

TABLE 1 Nitrogen. Top Bottom Nitrogen Steam Nitrogen pres. tube tubeflow rate pres. tank at inlet temp., temp., in interval (p.s.i.g.) pres.to dlgest F. F. calculated (ga. 52)l (p.s.i.g.) (ga. 40)l (ga. 43)l (ga.44)l lb. mole/min.

1Refers to gan inch in diameter on an average, was adjusted te 444 geillustrated in drawings. The flow mate of the chips, measuring about $4grams per minute of 0.D. (oven dry) wood. The flow rate was `control-ledso thut the residence time of any given chi-p within the digesterreactor ltubes was about 14 minutes in rtotal. measure 407 cubic centim-wash screen for a period of 4 minutes. Thereafter, the

utes -with hot Waiter and then minutes wilth cold water. Th

disse reiiner manufactured by Sprout Waldron in ya ii while debering thechips. The resulting pulp was placed in tin cans yand weighed within 0.1gram. The Samples overnight.

Example II Using the same apparatus, .a duplicate amount of chips and aduplicate amount of liquor as in Example I, another run was carried outdiffering only in that no noncondensable nitrogen was introduced intothe system. It was noted during this run that the temperatures indicatedon gauge 43 and 44 (upper and lower reactor tubes, respectively) wereapproximately the same, e.g., 350 F.

The pulps produced and collected in accordance with Examples I and IIwere formed into paper sheets which were subjected to a Mullen test, atear test and a ring crush and the results summarized in Table 2.

l Yield is obtained by dividing the oven dried pulp produced during the4 minute run, divided by the total oi oven dried wood fed to thedrgester during the same interval.

The Mullen Bursting Strength Test was determined using a B. F. PerkinsMullen Tester ranging from 0-120 p.s.i. in accordance with TAPPIStandard Method T403 TS-6-3.

eters per minute. The resulting pu p thickened on a to drain forapproximately 15 minutes and then centrifuged ulp was then crumbled andfive 200 ganst any change in moisture content The 1l uur iiow rate waswas blown into a flat bottom Itimed sample was washed for mine pulp wasthen put through a ovv of water achieving a complete washing n 80 meshscreen and allowed controlled rto -to approximately 20% congram sampleswere taken for polyethylene bag which was The 200 gram samples Were wereoven dried ait -110 can be seen, the brightness of the paper was higherby a sizeable factor, e.g., 8 points, (19.5-1 1.5) which approaches a1.00% improvement.

Example III In another set of runs, a liquor having the followinganalysis was utilized.

Grams per liter In this run, the same chips as before were utilized butat a flow rate of 434 grams per minute on the basis of oven dried wood,while the liquor flow rate was set at 391 cubic centimeters per minute.

Table 3 below lists the duration of the run in minutes (33) as well astime increments during which the steam pressure was taken on pressuregauge 52 together with the nitrogen tank pressure in p.s.i.g., thenitrogen pressure at the inlet to the digester tube '18 taken onpressure gauge 40 together with the temperature taken on gauge 43, thetemperature taken on gauge 44, the digester pressure taken on gauge 42and the nitrogen flow rate during the incremental time elements in poundmoles per minute.

TABLE 3 N itrogen pres Top Bottom Nitrogen Steam Nitrogen at inlet tubetube Dlgester ow rate pres Tank to digest temp., temp., pres. ininterval (p.s.i.g.) pres (p.s.i.g. F, (p.s.i.g.) calculated, (ga. 52)l(p.s.i.g) (ga. 40)l (ga. 43)l (ga. 44)l (ga. 42)l lb. mole/min.

l Refers to gauge illustrated in drawings. The tear resistance wasdetermlned on a standard Elmen Example 1V dorf Tearing Tester inaccordance with TAPPI Standard Method T414 M-49. The ring crush of thesamples was A run identical to that of Example III was conducteddetermined in accordance with TAPPI Standard Method 75 over the sametime interval and having the same read- TABLE 4 Control Example IIIExample lV Mullen (p.s.i.) 31 28 Tear (gram/16 sheets). 57 53 Ring Crush(lbs/y" x 6")- 58 52 GE photovolt brightness (percent) 18. 11. 5 Yield 166 63. 9

l Yield is obtained by dividing the oven dried pulp produced during the4 minute run, divided by the total of oven dried wood fed to thedigester during the same interval. It may be noted from Table 3 that thetop tube temperatures ran between 20 and 26 F. cooler than thetemperature in the bottom tube, demonstrating that paper of as good orbetter properties could be achieved with less rigorous pulpingtemperatures although at the same pressure.

The reason for the increased brightness when utilizing the system of thepresent invention, wherein a non-condensable gas such as nitrogen isintroduced into the primary digester reactor tube, is not known.

Applicant has thus found that digestion of the cellulose-yielding woodchip is improved where the first stage of the digestion is accomplishedat a lower temperature provided by reason of the padding atmosphereproduced by introducing a non-condensable gas, such as nitrogen,contemporaneous with the introduced wood chips and contemporaneous withthe introduction of the cooking liquor.

My invention is not to be considered limited to the production of pulpand/or fiber from wood chip and wood waste but, of course, is applicableto the production of pulp and fiber, for example, from any cellulosicstarting material such as flax, wheat and other straws, grasses,bagasse, linters, etc.

While only nitrogen has been utilized in the aforesaid examples, it willbe appreciated that air under certain circumstances, as well as othergases, may be substituted in whole or in part; their choice being basedupon a proper consideration of the digesting liquor, the nature of thecellulose-contributing substance, etc.; care being taken to select a gaswhich is relatively, e.g., substantially, noncondensable. Asubstantially non-condensable gas is one which tends not to condensewithin the digester tube reactor. Stated conversely, a relativelynon-condensable gas is one which tends to remain a gas within thetubular reactor and thus available to fulfill its function of precludingentry of steam into the inlet region of the tubular reactor. Thus, itwill be appreciated that oxygen will serve as a suitable relativelynon-condensable gas with certain pulp digesting cooks. Ammonia (NH3) isa suitable noncondensable gas in alkaline cooks while `gases such assulfur dioxide (SO2) and carbon dioxide (CO2) are suitablenon-condensable gases for acid cooks. The so-called combustion gases arealso candidate non-condensable gases, assuming appropriate care in thematching of combustion gas component and the nature and character of theparticular cook involved.

Having in mind (l) overall eiciency of pulping, regardless of thecharacter of the cook (alkaline or acid), (2) the optimum in propertiesof the ultimate sheet and (3) the chemical inertness of nitrogen; it ismost preferred to use nitrogen as the non-condensable gas in thedigestion of cellulosic substances in accordance with the presentinvention.

I claim: 1. The method of converting lignocellulosic material into pulpwhich comprises:

introducing said material into a first substantially horizontal chambertogether with a chemical adapted to digest the binding ligniningredients and encrusting substances holding cellulose fibers together,

introducing a non-condensable gas into said first chamber, said materialbeing resident in said first chamber for a period of time sufiicient toeffect impregnation,

passing said material from said first chamber directly into a secondsubstantially horizontal chamber,

introducing steam into said material and said second chamber adjacentthe point of introduction of said material into said second chamber tothereby raise the temperature in said second chamber to a level greaterthan the temperature in said first chamber,

controlling the pressure of said non-condensable gas in the firstchamber so that said first chamber is maintained at a higher pressurethan said second chamber thereby substantially prohibiting the migrationof the steam from said second chamber to said first chamber,

passing said material and steam through said second chamber at saidgreater temperature whereby digestion proceeds,

said material being resident in said second chamber for a period of timesuflicient to effect digestion,

and discharging said material.

2. The method as claimed in claim 1 wherein said noncondensable gas isnitrogen.

References Cited UNITED STATES PATENTS 2,858,213 10/1958 Durant et al162-237 X 2,963,086 12/1960 Green 162-237 3,294,625 12/ 1966 Gessner162-68 3,308,011 3/1967 Ross 162-68 S. LEON BASHORE, Primary Examiner'.

R. D. BAJEFSKY, Assistant Examiner.

U.S. Cl. X.R. 162-18, 68, 237

